FIELD OF THE INVENTION
This invention relates to silencers, or mufflers, and particularly those including corrosion resistant ferrous alloys such as stainless steel as the base material of fabrication.
BACKGROUND TO THE INVENTION
Silencers or mufflers are used in internal combustion engines to reduce noise levels; and exhaust emission levels when the silencer is fitted with a catalytic converter. Noise levels are reduced by use of various components including resonator chambers and dampener arrangements including baffles and so on. Catalytic elements are employed for reduction of exhaust emission levels, particularly of hydrocarbons and NOx.
Silencers or mufflers are typically made of a sheet metal, such as mild steel, as the base material of manufacture. In the automotive industry, and typically in two wheeler applications where silencers are a prominent element of the vehicle, the silencer is of particular aesthetic importance. To that end, an attractive or decorative surface treatment is usually applied to outer surfaces of the silencer. Surface treatments typically used include nickel chrome plating and heat resistant painting to internal parts which additionally serve to confer corrosion resistance on the silencer which is exposed to high temperatures and corrosive exhaust gases.
Where mild steel is used as the base material for the silencer, the inner parts of the silencer are also painted to provide corrosion resistance to hot exhaust gases. One of the problems observed with this type of surface coating is that, due to the high temperature of the exhaust gases, the inner parts of the silencer corrode after a very short service life when the base material is mild steel. Such corrosion leads, inevitably, to failure and consequential malfunctioning of the silencer. This causes nuisance in the form of additional noise and exhaust gas emissions for both owners and passers by. In addition, silencer replacement is required and this is a cost that would be better avoided.

A possible solution to the corrosion problem is provided by electroplating of the Inner parts of the mild steel silencer and, while this may be effectively carried out, the costs of manufacture are substantially increased and the problem of cost for the purchaser of the silencer remains. Besides, internal plating cannot fully solve the problem of corrosion owing to subsequent weld operations.
It would be desirable, therefore, to provide a silencer or muffler that has enhanced corrosion resistance to those components previously available and which, at the same time, has an attractive or aesthetically pleasing surface appearance that will promote sales of the silencer and the two wheel vehicle employing it.
SUMMARY OF INVENTION
With this desirable object in view, the present invention provides a silencer comprising a body having a visible, preferably aesthetic, outer surface and an inner surface wherein the body comprises a ferrous alloy, having a higher corrosion resistance to vehicle exhaust gases than mild steel, as a base material and at least the outer surface is electroplated, advantageously for corrosion resistance and aesthetic purposes.
Stainless steel is a ferrous alloy having higher corrosion resistance to vehicle exhaust gases than mild steel and a suitable grade of stainless steel is advantageously selected as base material for the silencer or muffler. Grades are classified, for example, in accordance with the ASTM system dependent on crystalline structure. A suitable grade of stainless steel for the silencer or muffler application is '409 grade' stainless steel. Other grades may also be suitable dependent on cost and corrosion resistance. The use of stainless steel, despite its well recognized corrosion resistance, is counter-intuitive for use as a base material where electroplating is required. That is because there is a problem of poor adhesion of metal coatings to stainless steel. The problem is particularly acute in the case of electroless nickel or nickel chrome plating which is the preferred form of plating to be adopted in accordance with the present invention. Plating failure will manifest itself in poor aesthetic appearance and poor corrosion resistance.

Adhesion is improved considerably through use of an acid activation or etching step, preferably following buffing, to prepare at least the outer surface of the silencer preform to receive a nickel coating. Acid activation may be conducted with hydrochloric acid in an electrolytic bath in which the silencer preform acts as the anode. One or more control parameters selected from the group consisting of acid concentration, time of treatment, temperature, current density and voltage are selected to optimize activation of the silencer preform surface and subsequent quality of electroplating of the silencer as measured by durability of the nickel plate or coating and, consequently, service life of the silencer. Acid concentration, temperature and time of treatment have been found to be particularly important by the present applicant.
Activation is followed by a 'nickel strike' process, for example the Woods nickel strike process, to form a nickel coating or plate on at least the outer surface of the silencer. The nickel strike process involves immersion of the silencer or muffler preform in an electrolytic bath containing an electrolytic nickel solution, such as a solution of hydrochloric acid and nickel chloride, the concentration of nickel chloride and hydrochloric acid being selected to enhance the adhesion of the nickel plate or coating. It is desirable to control the concentration of hydrochloric acid to as low a level as possible to minimise plating time. To this end, hydrochloric acid concentration should not exceed 100 ml/I solution and concentration in the range 80-90 ml/I solution is preferred. Nickel chloride concentration is preferably selected in the range 200-250 g/l of solution. Nickel anodes are then immersed in the bath, the silencer preform forming the cathode and the circuit is completed between them by the electrical power source being a DC power source. The quality of the nickel electrode material is important to achieving a suitably durable coating whilst conducting an efficient electroplating process. Electrolytic nickel is preferred and the nickel anodes should contain minimal sulphur. Control parameter(s) selected from nickel chloride concentration, hydrochloric acid concentration, time of treatment, temperature, current density and voltage are selected to optimize the nickel coating. Nickel chloride concentration, hydrochloric acid concentration, time of treatment and current density have been found to be particularly important by the present applicant.

The acid activation and nickel strike steps are advantageously performed in a single electrolytic bath. For acid activation, the silencer preform is maintained as the anode. The activation is performed with a suitable acid solution (such as hydrochloric acid and nickel chloride solution), conveniently the same electrolytic nickel or hydrochloric acid nickel-chloride solution to be used later for the 'nickel strike' stage. At the end of the activation stage, the polarity of the electrolytic bath is reversed and nickel plating - as above described - is conducted. The temperature, duration and current density parameters are optimized to achieve an effective nickel plating of the silencer preform. The current density may be the same for both activation and nickel strike stages or the current densities may be different for each stage.
Following the nickel strike treatment, the silencer may be subjected to further nickel and chrome plating stages which may be conventional in nature. One or more water rinse stages follows each electrochemical stage as removal of chemicals used at each stage is important to achievement of a desirable plating or coating quality particularly having regard to lustre and adhesion.
The electroplating provides good aesthetic appearance and corrosion resistance for the product silencer. The stainless steel interior of the silencer need not be subjected to electroplating and heat resistant painting may be adopted if further corrosion resistance is required. However, it is to be understood that electroplating of the inner surface of the silencer is not precluded and may be advantageous for some applications. Moreover, weld operations on stainless steel material require stainless steel weld rods advantageously implying that the welded joints are not prone to corrosion.
The silencer or muffler of the invention may be used in various applications and is advantageous for two wheeler, such as motorcycle, applications.
BRIEF DESCRIPTION OF THE DRAWINGS
The silencer or muffler of the present invention may be more fully understood by the following description of a preferred embodiment of the present invention made with reference to the accompanying drawings in which:

Figure 1 depicts a process flow sheet for a surface treatment process followed for a silencer in accordance with the prior art;
Figure 2 depicts a process flow sheet for a surface treatment process followed for a silencer in accordance with a first embodiment of the invention;
Figure 3 depicts a process flow sheet for a surface treatment process followed for a silencer in accordance with a first embodiment of the present invention and showing the control parameters involved;
Figure 4a depicts a schematic of an electrolytic bath for the acid activation stage used to manufacture a silencer in accordance with one embodiment of the present invention;
Figure 4b depicts a schematic of an electrolytic bath for nickel strike stage used to manufacture a silencer in accordance with one embodiment of the present invention;
Figure 5 depicts a perspective view of a silencer preform prior to final painting to produce a silencer in accordance with one embodiment of the present invention; and
Figure 6 depicts a sectional view along section line A-A of Figure 5 and showing the internal surfaces of the silencer preform.
DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
Referring first to Figure 1, the usual process flow sheet for electroplating of a silencer pre-form in accordance with the prior art is illustrated. The silencer pre¬form, made of mild steel, is pre-treated to remove contaminants and then the pre¬form is exposed to a series of electrolytic nickel baths in accordance with conventional practice, to plate nickel. Chrome plating follows and the silencer is ready for sale following painting of its inner surfaces with corrosion resistant paint. Unfortunately, silencers fabricated in accordance with this process flow sheet have an unsatisfactory service life as the hot exhaust gases to which the inner surfaces of the silencer are exposed cause the corrosion resistant paint to break down, a rapid corrosion rate follows and replacement of the silencer is necessary to avoid nuisance through excessive noise and exhaust emission levels.
Figures 2 and 3 address the process flow sheet for manufacturing a silencer in accordance with one embodiment of the present invention. The base material

selected for the silencer is stainless steel, of ASTM grade 409, and a silencer pre¬form 2 (as conveniently illustrated in Figures 5 and 6) is manufactured from this grade of stainless steel. It is this silencer pre-form 2 which is subjected to an electroplating process to deposit a nickel chrome plate on the outer surface 12 of the silencer pre-form.
The silencer pre-form 2 is first subjected to buffing and acid pickling steps to clean the metal surface for electroplating. However, unlike the mild steel silencer manufactured according to the process flow sheet of Figure 1, a specific nickel strike process is first required to deposit a nickel coating or plate on the silencer pre-form as nickel coating adhesion to stainless steel is problematic. The conditions under which the nickel strike process is conducted are therefore very important to ensure that acceptable adhesion of the subsequent nickel-chrome plate to achieve the desired corrosion resistance and aesthetic purposes. Accordingly, the control parameters for the nickel strike stage, and the prerequisite acid activation stage, are carefully selected, with reference to the form of the silencer, to optimize effectiveness of the electroplating process.
Both acid activation and nickel strike processes are conducted in the same electrolytic bath 4 which reduces space requirements for the process as well as time and hazard in moving treated silencer pre-forms from one bath to another. A solution of nickel chloride and hydrochloric acid is prepared for the bath 4. This solution contains 200-250 g/l nickel chloride and 80-90 ml/I MCI. The HCI concentration is maintained below 100 ml/I to prevent HCI concentration adversely affecting the plating duration or other aspects of the electroplating process. The bath 4 is maintained at a temperature of between 50 and 55°C through provision of a suitable heater element. The silencer pre-form 2 acts as one electrode and electrolytic nickel 3 acts as the other electrode 3. It may be understood that nickel substitutes for electrolytic nickel may be available for use as the nickel electrode but selection of such substitutes may adversely affect the process and adhesion of the nickel chrome plate. Sulphur containing nickel electrodes are particularly advantageously to be avoided. The electrodes 2 and 3 are connected through a DC power source 7.

In the acid activation stage, the silencer preform 2 acts as the anode having positive polarity (see Fig. 4(a)), the stainless steel being etched and activated by the nydrochloric acid - nickel chloride solution, making the stainless steel more receptive to subsequent nickel and chrome electroplating stages. Activation proceeds for 1-3 minutes with a current density of 25-30 A/ft^ being applied.
The nickel strike stage follows. In this stage, polarity is reversed and the electrolytic nickel electrode 3 becomes the anode (See Fig 4(b)) and the silencer pre-form 2 becomes the cathode that is subject to plating or coating with nickel metal ('nickel strike'). Nickel strike proceeds for 3-6 minutes with a current density of 25-30 A/ft^ being applied.
Following this stage, a nickel coating is formed on the outer surface of the silencer pre-form and further nickel and chrome plating stages may follow a water rinsing/drag out stage. The preferred sequence of further nickel plating stages is semi-bright nickel/tri-nickel/bright nickel in accordance with conventional practice. Chrome activation and chrome plating stages follow water rinsing/drag out step(s). The conditions for these subsequent steps are summarized in the process flow sheet of Figure 3.
The silencer pre-form 2, manufactured following the process flowsheet illustrated in Figures 2 and 3 is shown in Figure 5. The outer surface 12 of the silencer pre-form 2 is now electroplated providing a finish that is both corrosion resistant and aesthetically attractive. The inner surfaces of the silencer pre-form 2, , and for example as marked 15 in Figure 6, are painted with a heat and corrosion resistant paint to complete silencer 2 manufacture. The painting operation for the inner surfaces may be carried out using a conventional process such as flow coating. The silencer 2 is then ready for sale, advantageously for use in motorcycles.
Further advantages of the silencer of the present invention may be demonstrated by reference to the following example:
EXAMPLE
A stainless steel silencer for use on a motorcycle was electroplated with nickel-chrome plating following the above described steps. The corrosion resistance

of the nickel chrome electroplated stainless steel silencer was doubled over the service life of the nickel chrome electroplated mild steel silencer. The electroplated mild steel silencer could pass up to 24 hours following the CASS (copper accelerated salt spray test) and the electroplated stainless steel silencer passed up to 48 hours.
The following table indicates the quality parameters achieved:

Sr. No. Test Method of test Achieved specs
1 Ra Value After buffing Perthometer Instrument Body Muffler * 0.10 -0.25 micron

Exhaust Tube 0.20 - 0.45 micron
2 Nickel Thickness By Step Instrument, Potentiometric method 25 - 30 Micron
3 Bend Test 180 degree bend on conical mandrel, dia 10 mm No crack, No peel off
4 Adhesion test Filing edge at 45 degree with coarse file No Nickel film lifting from substrate
5 Heat Test Heating by holding at 400 degree eels, for 30 mins followed by immediate quenching No blister. No peel off
6 CASS test IS:5528-1985 Subjected to minimum 48 hrs. exposure in CASS test chamber Corrosion rating no 8 achieved should not be less than no.8 for all significant surfaces ( to get exact details)
Modifications and variations to the silencer or muffler of the present invention may be apparent to the skilled reader of this disclosure. Such modifications and variations are deemed within the scope of the present invention.

WE CLAIM:
1. A silencer comprising a body having a visible outer surface and an inner
surface wherein the body comprises a ferrous alloy, having a higher corrosion
resistance to vehicle exhaust gases than mild steel, as a base material, and at least
the outer surface is electroplated.
2. The silencer of claim 1 wherein said ferrous alloy is stainless steel.
3. The silencer of claim 1 or 2 wherein the outer surface is electroplated with a
nickel chrome plating.
4. The silencer of any one of the preceding claims wherein an inner surface of
the silencer is not subjected to electroplating.
5. A motorcycle comprising a silencer as claimed in any one of the preceding
claims.
6. The silencer substantially as hereinbefore described with reference to the
accompanying drawings.